Support `prefers-reduced-motion` media query

When clicking left/right paddles, the current implementation is too fast for those with motion-triggered vestibular spectrum disorder. Some users will likely experience distraction or nausea from the animation. `flickity` should attempt to support the `prefers-reduced-motion` media query.

```js
const mediaQuery = window.matchMedia('(prefers-reduced-motion: reduce)');
mediaQuery.addEventListener('change', () => {
console.log(mediaQuery.media, mediaQuery.matches);
// Stop JavaScript-based animations.
});
```
```css
@media (prefers-reduced-motion: reduce) {
// Override CSS-based animations
}
```
Source: https://developers.google.com/web/updates/2019/03/prefers-reduced-motion

Alternate approaches:
• Slow down animations considerably.
• Fade in/out into the next shelf pagination, instead of `smoothScroll`.

Start by tracing the left/right paddle animation and the smoothScroll path mentioned in the issue, then review how Flickity handles JavaScript-based movement. Done means the carousel responds to the prefers-reduced-motion preference, including the listed alternative behavior, without disrupting normal animation for other users.

Written by the indexing model from the issue text.